Photo detector system with dual mode capability

ABSTRACT

A photo detector system is described which has dual modes of operation under both dim and bright illumination conditions. Optics image incoming illumination directly upon an array of semiconductor detector diodes as well as upon a photo cathode. When bright light is present the optical image is focused upon the diode detector array and no high voltage is applied to the photo cathode. The light will not damage the photo cathode since the high voltage is off. Also, no electrons are accelerated toward the photo diode detectors. Thus, during bright light operation, the signal current is caused primarily by the direct effect of light on the diode detectors. In dim light, high negative voltage is applied to the photo cathode and the optical image is focused on the photo cathode. The electron optic design causes the electrons emitted from the photo cathode to be imaged onto the same diode detectors that are used during bright light operation. These electrons strike the detectors with a high velocity induced by the applied voltage. The signal current due to this electron bombardment is therefore many times greater than the current due to the dim light striking the diodes directly. Detector response to dim levels of illumination is thus increased while retaining normal operational capability under bright illumination conditions.

. United States Patent 1191 Kalitinsky et al. May 22, 1973 [54] PHOTODETECTOR SYSTEM WITH [57] ABSTRACT DUAL MODE CAPABILITY A photo detectorsystem is described which has dual [75] Inventors: Andrew Kalitinsky, LaJolla; James modes of operation under both dim and bright illu- W.'Crooks, Jr., San Diego, both of mination conditions. Optics imageincoming illumina- Calif. tion directly upon an array of semiconductordetector [73] Assignee: General Dynamics Corporation, San diodes as wellas upon a photo cathode. When brlght Diego Calif. light 1s present theoptical image is focused upon the diode detector array and no highvoltage 1s applled to [22] Flled: r 1972 the photo cathode. The lightwill not damage the 2 App] N 241,932 photo cathode since the highvoltage is off. Also, no

electrons are accelerated toward the photo diode de- 52 us. or ..250/213v'r, 250/220 M, 313/65 AB T22 g lgi s l i g 12E322? 51 Int. Cl. .1101,31/26, HOlj 31/50 2 diode deteftors y y e [58] Field of Search ..313/65AB; e

250/220 1 T In dim light, high negative voltage is applied to the photocathode and the optical image is focused on the References Cited photocathode. The electron optic design causes the UNITED STATES PATENTSelectrons emitted from the photo cathode to be imaged onto the samediode detectors that are used 2,213,548 9/1940 [81118 ....250/220Mduring operation These electrons strike 29031596 9/1959 Reed "250/213 VTthe detectors with a high velocity induced by the ap- 3,322,955 5/1967Desvrgnes ..250/213 VT plied voltage The Signal current due to thiselectron Primary Examiner Archie R Borchelt bombardment 1s thereforemany times greater than the Assistant ExaminerT. N. GrigsbyAttorney-Martin LuKacher current due to the dim light striking thediodes directly. Detector response to dim levels of illumination is thusincreased while retaining normal operational capability under brightillumination conditions.

34 Claims, 7 Drawing Figures PATENTEL HAY 2 21975 SHtU 5 UP 5 PHOTODETECTOR SYSTEM WITH DUAL MODE CAPABILITY The present invention relatesto the radiant energy detection systems and particularly to detectionsystems with the capability of augmenting or intensifying radiant energysignals under low intensity (e.g., dim light) conditions.

The invention is especially suitable for use in image detectors whichprovide electrical signals in response to images. The detector providedby the invention also has applications in aids for blind persons,devices for detecting objects in the dark (viz., infra-red scopes) andin camera systems for television purposes as well as in any otherapplications where detection of radiant energy at different levels ofintensity is required.

Oftentimes it is necessary to sense and detect images in both bright anddim illumination. By illumination is meant visible illumination (i.e.,light), invisible illumination such as infra-red, ultra-violet and otherfrequencies in or near the optical wavelengths, and other radiant energyillumination. Typically, intensification of the image is accomplishedelectronically after the image is translated into the form of electronicsignals. More often, completely different detectors must be provided andused under dim illumination conditions. The increase in the sensitivityof dim illumination responsive detectors is limited by thecharacteristics of photo-sensitive materials used therein. Detectorswhich operate by increasing the amplitude of the transduced electronicsignals are subject to the influence of noise as well as the complexitybrought about by the addition of the requisite electronic equipment.

It is a feature of this invention to provide a photo detector systemwith the capability of augmenting the response to images received underdim light conditions in the photo detector itself. Normal operationcapability of the photo detector is retained and the apparatus may beused with light or dim illumination with a minimum of readjustment.

Accordingly, it is an object of the present invention to provide animproved photo detector system.

It is a further object of the invention to provide photo detectorapparatus capable of operation with high level and low levelillumination using the same photo detector components.

It is a still further object of the present invention to provideimproved apparatus capable of intensifying images electronically.

It is a still further object of the present invention to provideimproved image intensification apparatus wherein intensification isaccomplished within the photo detector device itself.

Briefly described, apparatus embodying the invention which is capable ofintensifying images, includes as its components, a photo electricelement. Radiant energy from the image may be imaged directly on thisphotoelectric element. A photo emissive element is also provided. Theradiant energy from the image may also be imaged upon this photoemissive element. Electron optics are provided for imaging the electronsemitted by the photo emissive element upon the photo electric element.The image produced by the electrons impinging upon the photo electricelement is thereby intensified. It is believed that such intensificationresults from the phenomenon known as electron bombardment conductionwhereby high velocity electrons cause a much larger current to flow inthe photo electric element than is caused by illumination directlyreceived by the element. Under bright illumination conditions the imagesare focused at the photo electric element, while under dim illuminationconditions the images are focused on the photo emissive element and theelectron optics are used to accelerate the electrons; thus producing anelectron image which is translated by the photo electric element intothe relatively high level signals corresponding to signals which areproduced under bright illumination conditions.

The invention itself, both as to its organization and method ofoperation, as well as additional objects and advantages thereof, willbecome more readily apparent from a reading of the following descriptionin connection with the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of a photo detector tube embodying theinvention;

FIG. 2 is an end-view of the tube shown in FIG. 1;

FIG. 3 is a cross-sectional view of a photo detector system embodyingthe tube shown in FIGS. 1 and 2, together with optics focusing the imageto be detected;

FIGS. 4a and 4b are simplified cross-sectional views of a detector tubein accordance with another embodiment of the invention. In operation inthe augmented (e.g. dim light) and unaugmented (e.g. bright light) modesrespectively;

FIG. 5 is a schematic diagram of the photo detector diode arraycontained in the tube shown in FIGS. 1 and 2, together with associatedelectronic circuitry; and

FIG. 6 is a simplified, diagramatic, perspective view of aphoto-detector system utilizing a line array of diode detectors to scanelectronically in one dimension and a mirror to optically scan in asecond dimension.

Referring particularly to FIGS. 1 and 2, there is shown a photodetectortube having an envelope'l0 which may be made of glass. A window 12 madeup of two glass plates, 14 and 16, are located at the lower end of theenvelope as viewed in the drawing; the tube being capable of orientationin any direction or operational position. Another plate 18 is located atthe upper end of the envelope. The cylindrical wall 20 of the envelope10 is sealed to the plates 12 and 18 by cusp shaped rings or headers 22and 24. The lower lip 26 of the header 22 extends radially inward alongthe surface of the plate 14, leaving an opening at its center. Theenvelope 10 is evacuated and glass to metal seals are used in connectingthe headers 22 and 24, and other metallic members to the glass portionof the envelope. Conventional techniques used in the fabrication ofvacuum tubes may be employed for this purpose. In the event that thetube is to be used in a vacuum environment (e.g., in space) the window12 may be dispensed with.

Supported on a sheet 28, of insulating material, as by being cementedthereto, is an array 30 of semiconductive photo diodes. These diodes, asmay be observed in FIG. 2 are equally spaced from each other alongrectangular (x-y) coordinates. The diode array 30 may be fabricated inaccordance with conventional semiconductor techniques by growing ordepositing on a substrate of non-conductive material, such as ceramic,P-N junctions of silicon semi-conductive material which is photoconductive. Thus, illumination falling on one or more of these diodes inthe array 30 will produce electronic activity and conduction across thejunction. Individual leads 32 are made to each diode. There may be twoleads per diode. However, preferably, a single layer of conductivematerial may be deposited on the substrate so as to make contact withone side of each junction and individual leads 32 provided so as to makecontact with the other side of each junction. The leads 32 are broughtout through the envelope, the metal to glass seals being preserved so asto maintain the vacuum, and the leads connected to terminals 34. Theseterminals 34 may be mounted on the flange of a metal bracket 36 which isa tubular spool in shape and attached to the rear end 18 of the.envelope. Conducting terminals 34 and also terminal 40 may be attachedby non-conducting material 38 inserted in the flange of 36 to formfeed-through terminals, such that each of the terminals 34 and also 40terminal is insulated from the other terminals and other parts of thetube. Thus, a ring of terminals is disposed on the support, 36, near theouter rim of the flange thereof. One of the terminals 34 is connected tothe common side of each of the diodes in the array 30. A terminal 40 inthe ring of terminals 34 is provided for a ground connection. Thisterminal 40 is connected to the header 24 and to an aperture plate 42 ofconductive material disposed around the forward surface of the plate 16which forms part of the window 12. The aperture in the plate 42 permitsthe passage of illumination into the tube. A conductive transparentcoating, not shown, may be used to cover the outer surface of window 12,connecting tov ring 42, to prevent electrostatic charges from buildingup on the window surface.

A photo emissive element 44 in the form of a layer of photo emissivematerial is placed as by being deposited on a portion of the innersurface of the window 12. The photo emissive material may becesium-antimony (C S or some other material which is photo emissive atthe wavelength of the illumination of interest viz., the illuminationwhich is to be detected by the photo detector system). Preferably, thelayer of photo emissive material 44 is partially transparent to theillumination or at least substantially translucent thereto. Accordingly,a translucent photo emissive material such as a thin layer ofcesium-antimony is desirably used, or the layer may be made especiallythin so as to be penetrated by the illumination. As may best be observedin FIG. 2 the portion of the window on which the photo emissive materialis disposed, is a semi-circular area offset to one side of the centralaxis of the envelope 10. Alternatively, if the photo cathode is madesufiiciently transparent to allow the required fraction of light toreach the photo diodes, the full window may be covered and light maypass through the photo cathode to the photo diodes for operation in thebright light mode. The center of the array 30 is directly on this axisand the half of the array which is visible through the window 12 mayalso be observed in FIG. 2. The layer of photo emissive material 44extends to and is in contact with the lip 26 of the lower header 22. Asource of negative (with respect to ground) high voltage, say -20 Kv isapplied to the header 22 by way of a high voltage cable 46. The upperportion 48 of this cable may be shielded so as to prevent induction ofany stray noise to the leads 32 from the photo diodes in the array 30.The photo emissive element 44 thus acts as a photo cathode. Whenilluminated, this photo cathode 44 emits electrons which, because thephoto cathode 44 is at a much more negative potential than the region ofthe envelope in which the diode array 30 is located, (note that theupper header 24 is connected to ground and that the diode array is at avoltage equal to or near envelope through vacuum type seals. Resistors62 are connected between the lower header 22 and the baffle 50, betweensuccessive ones of the battles S0 and 60,

and between the baffle 60, and the upper header 24. These resistors 62provide a resistor voltage divider through which proper potentialsderived from the source which is connected to the cable 46 to establisha uniform accelerating field for the electrons from the photo cathode.

The envelope 10, the periphery of the window 12, the rims of the baffles50 to 60, the resistors 62, the lower portion of the cable 46, and intotal all of the portions of the tube which are of high voltage, areencased in a body of insulating material 64. This body of insulatingmaterial 64 may be applied by locating the envelope and theaforementioned parts thereof, in a mold, and filling the mold withencapsulating material such as epoxy resin, which is allowed to hardenso as to form the body 64. A metal tube 66 encompasses the body 64. Thistube 66 may be in two parts, namely an open ended flanged cap 68 whichis secured in a spring fitting 70 which encompasses the upper portion ofthe body 64.

As shown in FIG. 3 the detector tube is surrounded by a pair ofdeflection coils 70 and 72 and by a focusing coil 74. The coil assemblyis surrounded by a tube 76 of magnetic material, such as soft iron whichacts as a magnetic return path and a shield. The casing tubes 66 and 68are of non-magnetic material, such as copper. The focusing coil 74serves to focus the electron image produced by the operation of thephoto cathode 44 on the surface of the diode matrix, and particularly onan individual diode of that matrix. In other words the electrons emittedby each small area on the photo cathode are formed into a beam which isfocussed to impinge on a corresponding small area at the upper end ofthe tube. Thus there is a small area on the photo cathode whichcorresponds to each diode in the diode matrix when a particularcombination of voltages and magnetic field is applied to the tube.

The deflection coils 70 and 72 deflect the electron beams across thediode matrix and thus can change the area on the photo cathode thattransmits electrons to each particular diode. This allows the selectionof the area on the photo cathode that is to be focussed on the diodesand is particularly important when only a portion of the window iscoated with photo cathode material. Furthermore, various portions of thephoto cathode can be made to have different properties. For example,parts of the photo cathode can be made sensitive to various frequenciesof radiation by varying the material of which the photo cathode is madeor by placing color filters 13 over selected portions of the photocathode. By proper adjustment of magnetic fields and voltage on the tubeit is also possible to change the size of the photo cathode area thatemits the electrons that are focussed on the diode array and thus changethe size (or magnification) of the image being detected.

As shown in FIG. 5 the photo detector diode array contains several rowsof individual diodes 78, three of which, 78, 80 and 82 are illustratedby way of example. All of the diodes are back biased by means of abattery 84 and the anodes of the diodes are connected through acomparator 86 to the input of a preamplifier 88. The comparator may be acommutator circuit which is operated by clock pulses from a clock pulsesource 90 so as to successively enable the connection of each diode on arow by row basis to the input of the preamplifier 88. In other words,the anodes of the diodes in the first row 78 will be connected one afterthe other through the comparator to the input of the preamplifier. Thenthe diodes in the next row 80 will be connected one after the other.Successive rows of diodes will similarly be connected, until finally thediodes in the last row 82 are successively connected to thepreamplifier.

The image detected on the diode array may be displayed usingconventional cathode ray tube display techniques utilized in televisionor by means of electronic printer such as used for transmission ofpictures by radio or wire or by an array of light producing elementssuch as filament lamps or light emitting diodes or by other means. Thecathode ray tube display has been chosen as an example.

In order to synchronize the scanning of the beam in a cathode ray tubedisplay device with the diode which is then being gated by thecomparator, horizontal and vertical deflection signals are generated bya horizontal counter 92 and a vertical counter 94. The horizontalcounter has a capacity to count up to a number corresponding to thenumber of diodes in each row of the array 30. When this number isreached, the counter 92 resets itself and applies a pulse to thevertical counter 94. The vertical counter counts a number correspondingto the number of diodes in each column of diodes in the array. When thecount equal to this number is reached, the vertical counter then resetsitself. A horizontal deflection generator 96 and a vertical deflectiongenerator 98 are connected to each of the stages of the counter and areoperative to decode the count stored therein into an analog deflectionsignal which is applied, in the case of the horizontal deflectiongenerator 96 to the horizontal deflection coil of a cathode ray tubedisplay device and in the case of the vertical deflection generator 98to the vertical deflection coil of the display device. The video signalis applied to the control grid of the cathode ray tube display device.Accordingly, the gating of the diodes in the array will be synchronizedwith the deflection of the electron beam in the display cathode raytube.

It is also possible to scan in one direction optically and in the otherdirection electronically. Referring to FIG. 6 a lens 130 is used toproject a scene onto the photosensitive surfaces of a detector tube 132which is similar to the tube shown in FIGS. 1 3. Tube 132, instead of amatrix array, incorporates a single row or line array of photo diodes134 and a photo cathode 136. For simplicity, the lens 130 is shownfocussed on photodiode line array 134 only, but it can also be focussedon the photocathode 136. Optical scan is accomplished by rotating amirror 138 on axis 140 by means of actuator 142 (e.g., a solenoid orservo motor). This causes the projection 144 of line array 134 to moveacross the scene being scanned in the vertical direction as indicated byarrow 146. Scanning in the horizontal direction is accomplished bysequentially commutating the diodes of line array 134 similarly to thescan of one line of the two-dimensional array 30 of FIG. 5. Framesynchronization is provided-by the scan start detector 148 whichcontains a light source, photo cell and suitable optical system. Whenmirror 138 moves past a predetermined angular position, it reflectslight from the light source back into the photocell and generates theframe start signal. Electronics 150 similar to those in FIG. 5 are usedto synchronize the horizontal line scan and to provide the video outputsignal.

Optical scanning in both directions (horizontal and vertical) can beused in accordance with the invention. In that event, only a singlediode or a continuous photo detective surface may be used in lieu of thearray 30. The optical system may include mirrors for scanning the objectto be detected, such that a small aperture is effectively scanned acrossthe photo-sensitive area. Synchronizing signals from the opticalscanning system such as, for example, pulses obtained at the end of eachhorizontal scan and at the end of each frame may be provided and,combined with the signal from the photo detector, used to reconstructthe image of the object being scanned, either directly or after beingrecorded, as on magnetic tape, through the use of television or othervideo recording techniques.

Returning to FIG. 5 the signal from the preamplifier is applied to anoperational amplifier 100. A gain at the amplifier may be adjusted bymeans of a potentiometer 102 and the threshold (viz., the lowest signallevel) of the amplifier may be adjusted by means of a potentiometer 104,which contains a threshold adjust voltage from a source indicated at +c.The output from the amplifier is a video signal which may be used toreconstruct the image detected by the photo detector system.Synchronizing signals obtained from the horizontal and vertical counters92 and 94 may be combined with this video signal when the image is to bereconstructed through the use of television techniques (viz., on acathode ray tube). FIG. 3 also illustrates the operation of theapparatus under normal, bright conditions of illumination and under dim,low level illumination conditions. For low level, dim conditions, a lenssystem illustrated by a condenser lens 104 focuses the image to bedetected on the photo cathode 44. The negative potential is then appliedby way of the cable 46 and the electron gun, made up of the headers 22and 24 and the baffles 50 to 60 operate as an electron optical system todirect the electrons emitted by the photo cathode so that they impingeupon the surface of the array 30. It is believed, and it should beunderstood that the invention is not predicated upon any theory ofoperation, that the high velocity electrons produce electron conductionwithin the semi-conductor, as for example, by reducing the work functionof the junction. The current then flows and develops a sufficientpotential to overcome the back bias as may be produced by the battery 84(see FIG. 5). The signal is proportional to the intensity of electronbombardment (viz., the electron current in the beam of electronsfocussed on the photo detector diode). Inasmuch as the photo emissionfrom the photo cathode is proportional to the intensity of light, theelectron induced current is proportional to the illumination. Thephenomenon referred to above has been called electron bombardmentconduction.

During normal, bright light conditions, an optical image is focussed byreadjusting the lens 104 or through the use of another lens 106 to focusthe image directly on the photo detector array '30. The image may befocussed on a single diode or upon the entire array which then may becommutated as described above, in connection with FIG. 5, to provide avideo signal output corresponding to the image which is to be detected.Inasmuch as the signal output from the photo detector array is enhancedby virtue of the electron bombardment conduction during the dim lightmode, the output signals both for bright light and dim light modes, maybe substantially the same in magnitude, thus affording dual modedetection capability within the photo detector apparatus itself andwithout the need for external modifications or separate low or dimillumination level devices.

Referring to FIGS. 4A and 43, there is shown a photo detector apparatusembodying the invention which is operative without refocussing of theoptical image from the photo detector array 30 on to the photo cathode44 when changing from the bright illumination to the dim illuminationmode of operation. The tubes shown in FIG. 4 and FIGS. 1 and 2 aresimilar and like parts have been identified with like referencednumerals. The illustration has been simplified in that the cable 46 andthe connections to the headers 22 and 24 and to the baffle electrodes50, 52, 54, S6 and 58, are not shown. It will be noted that the header24 has an inward, tapered extension 1 10 thereby eliminating one of thebaffle electrodes and simplifying the electron gun structure.

It would also be pointed out that the tube structure cross-section neednot be circular but may be elongated in one dimension if desired, toaccommodate a long array. The view shown in FIG. 4A and 4B could applyto the narrow dimension of the tube and the tube could be extended asdesired in the dimension perpendicular to the plane drawn.

in order to illuminate the photo cathode without refocussing, the samelens (not shown) is adapted to illuminate a mirror 1 12. The photocathode 44 is laterally offset from the mirror 112 on opposite sides ofthe axis of the tube. Also the mirror 112 is disposed between the photodetector array 30 and the photo cathode 44 such that the optical pathfrom the lens to a focus, either at the photo detector array, or at thephoto cathode 44 is the same. The mirror may be attached as shown to oneof the battles 54 or otherwise to the wall of the envelope 10. Thisarrangement has the further advantage that the photocathode need not betransparcut.

The augmented or dim light mode of operation is obtained by applying theaccelerating voltages to the headers and baffle electrodes, as wasdiscussed above in connection with FIGS. 1 through 3. The illuminationimaged at the photo detector 44 results in the emission of electrons.Instead of a focussing coil, a pair of bar magnets 116 and 118 areprovided. This pair of bar magnets extend longitudinally along the axisof the tube and are tilted so as to deflect the electron beam, as wellas to focus it, as shown by the dash line 120.

P16. 48 illustrates operation in the bright or normal illumination mode.Although light will be reflected from the mirror 112 to the photocathode, any electrons which are emitted from the photo cathode will notreach the photo detectors in sufficient number to effect conduction inthe photo detector diodes, since the high voltage is turned off in thisoperation mode. The illumination reaching the photo cathode 44 may bepartially reflected therefrom and from the window, 12.

However, such reflected illumination will be scattered by the baffles 50to 58 and by the 110 so as not to interfere with the detection of thedirect illumination by the array 30.

From the foregoing description it will be apparent 10 that there hasbeen provided improved photo detector apparatus capable of dual modes ofoperation, utilizing essentially the same components for both modes ofoperation. The detection of images and objects and other photo detectionpurposes, both for dim illumination and bright illumination, are therebyprovided within the apparatus itself and particularly with the samephoto detector tube. While two embodiments of the photo detector tubehave been described herein, it will be appreciated that variations andmodifications will be apparent to those skilled in the art.

What is claimed is:

1. Apparatus for detecting radiant energy from a source of radiantenergy which comprises:

a. a photo electric element for providing an electric signal whenirradiated by radiant energy from said source and by electrons,

b. a photoemissive element for emitting electrons when irradiated byradiant energy from said source, and

c. means for irradiating said photoelectric element with electronsemitted by said photoemissive element.

2. Apparatus for intensifying images which comprises:

a. a photoelectric element,

b. means for imaging radiant energy from said image upon saidphotoelectric element,

c. a photoemissive element,

d. means for also imaging radiant energy from said image upon saidphotoemissive element, and

e. electron optical means for imaging the electrons emitted by saidphotoemissive element upon said photoelectric element whereby tointensify said image at said photoelectric element.

3. The invention as set forth in claim 2, wherein said means for imagingsaid radiant energy images upon said elements includes optical means forfocussing said images upon either said photoelectric element or saidphotoemissive element.

4. The invention as set forth in claim 3 wherein said photoelectricelement and said photoemissive element are spaced from each other alongthe path of said radiant energy.

5. The invention as set forth in claim 4, wherein said photoelectricelement and said photoemissive element are also laterally offset fromeach other, said photoemissive element being disposed ahead of saidphotoelectric element in the direction of propagation of such energy.

6. Photoelectric detector apparatus which comprises:

a. a photoelectric transducer,

b. said transducer being disposed directly to receive illumination to bedetected,

65 c. a photoemissive element in spaced relationship with saidphotoelectric transducer, d. said element also being disposed directlyto receive illumination to be detected, and

e. electron optical means operative to direct electrons resulting fromsaid illumination to said transducer for augmenting the effect of saidillumination at said transducer.

7. The invention as set forth in claim 6 including means for renderingsaid electron optical means operative whereby to operate said apparatusselectively in augmented and normal modes.

8. The invention as set forth in claim 6 wherein said electron opticalmeans includes means for accelerating the electrons emitted by saidphotoemissive element so that said electrons bombard said transducer toinduce conduction therein.

9. The invention as set forth in claim 8, wherein said electron opticalmeans further includes means for converging said electrons into a beamfocused at said transducer.

10. The invention as set forth in claim 9 wherein said electron opticalmeans further includes means for deflecting said beam across saidtransducer.

11. The invention as set forth in claim 9 wherein said transducer is asemiconductive photodiode.

12. The invention as set forth in claim 10 wherein said transducercomprises a substrate disposed in the path of said beam and an array ofsemiconductive photodiodes disposed in side by side relationship on saidsubstrate.

13. The invention as set forth in claim 6 wherein said photoemissiveelement is a photocathode, and said electron optical means, includesmeans for applying voltage to said photocathode to make saidphotocathode more negative than said transducer.

14. The invention as set forth in claim 13, wherein said electronoptical means includes a plurality of electrodes disposed in spacedrelationship between said photocathode and said transducer, and meansfor applying negative voltages of successively higher magnitude to saidelectrodes with the highest of said voltages being applied to the one ofsaid electrodes closest to said photocathode.

15. The invention as set forth in claim 14 wherein said transducer has asurface, and wherein said electron optical means includes a coildisposed around said electrodes for converging said electrons from saidphotocathode into a beam focused at said transducer surface.

16. The invention as set forth in claim 14 wherein said electron opticalmeans includes a pair or array of permanent magnets, spaced laterallyfrom each other, said transducer and said photocathode being disposedadjacent opposite ends of the region between said magnets, said magnetsdefining a focussing field for electrons emitted by said photocathode.

17. The invention as set forth in claim 16, wherein said photocathode isdisposed along an axis extending longitudinally through said region,said photocathode being offset from said axis.

18. The invention as set forth in claim 17 including a mirror disposedbetween said photocathode and said transducer and offset from said axis,said mirror reflecting said illumination to said photocathode.

19. The invention as set forth in claim 18 wherein said transducer hasphotoconductive material having a surface, and wherein said electronoptical means includes means for scanning said focussed electrons oversaid surface.

20. The invention as set forth in claim 6 further comprising an envelopehaving a window for the passage of said illumination at one end thereof,said transducer being disposed at the end of said envelope opposite fromsaid window, said photoemissive element being a photocathode disposed onthe inner surface of said window, said electron optical means comprisingan electron gun within said envelope.

21. The invention as set forth in claim 20 wherein said photocathode isa layer of photoemissive material.

22. The invention as set forth in claim 21, wherein said layer ispartially transparent to said illumination.

23. The invention as set forth in claim 21, wherein said transducer islocated on the axis of said envelope and said photocathode is disposedupon a portion of said window which is laterally offset from said axis.

24. The invention as set forth in claim 21, including first lens meansfor focussing said illumination at said transducer, and second lensmeans for focussing said illumination at said photocathode.

25. The invention as set forth in claim 20, further comprisingdeflection coil means encompassing said envelope, and focussing coilmeans also encompassing said envelope.

26. The invention as set forth in claim 25, wherein said transducer isan array of semiconductive photodiodes, and leads to each of the diodesin said array extending from said diodes through said envelope at saidone end thereof.

27. The invention as set forth in claim 20 wherein said electron guncomprises a plurality of conductive electrodes disposed within saidenvelope, said electrodes being coaxial with the axis of said envelopes,one of said electrodes being disposed upon said window and in contactwith said photocathode, said electrodes being spaced from each other ina direction longitudinally along said axis, means for applying a firstnegative potential to said one electrode and negative potentials ofsuccessively lower magnitude than the magnitude of said one potential tothe other ones of the other of said electrodes which are spacedsuccessively closer to said transducer.

28. The invention as set forth in claim 27 including a cylinder ofencapsulating material surrounding said envelope.

29. The invention as set forth in claim 28 including bar magnetsencapsulated in said material and diametrically disposed in thedirection of the axis of said envelope and diametrically opposite eachother on opposite sides of said axis.

30. The invention as set forth in claim 29 wherein said photocathode isdisposed to one side of said envelope, a mirror disposed within saidenvelope and spaced away from said window toward said transducer and tothe opposite side of said envelope forreflecting illumination to saidphotocathode, said bar magnets being tilted with respect to the axis ofsaid envelope for establishing a magnetic focussing field for focussingelectrons emitted from said photocathode at said transducer.

31. The invention as set forth in claim 20 including a means forfocussing electrons emitted from different portions of said photocathodeupon said transducer.

32. The invention as set forth in claim 31 wherein said focussing meanscomprises deflection and focussing coil means surrounding said envelope.

33. The invention as set forth in claim 31 including means for providingsaid portions with different spectral characteristics.

34. The invention as set forth in claim 33 wherein said spectralcharacteristic providing means comprises filters interposed in the pathof illumination to said photocathode.

1. Apparatus for detecting radiant energy from a source of radiantenergy which comprises: a. a photo electric element for providing anelectric signal when irradiated by radiant energy from said source andby electrons, b. a photoemissive element for emitting electrons whenirradiated by radiant energy from said source, and c. means forIrradiating said photoelectric element with electrons emitted by saidphotoemissive element.
 2. Apparatus for intensifying images whichcomprises: a. a photoelectric element, b. means for imaging radiantenergy from said image upon said photoelectric element, c. aphotoemissive element, d. means for also imaging radiant energy fromsaid image upon said photoemissive element, and e. electron opticalmeans for imaging the electrons emitted by said photoemissive elementupon said photoelectric element whereby to intensify said image at saidphotoelectric element.
 3. The invention as set forth in claim 2, whereinsaid means for imaging said radiant energy images upon said elementsincludes optical means for focussing said images upon either saidphotoelectric element or said photoemissive element.
 4. The invention asset forth in claim 3 wherein said photoelectric element and saidphotoemissive element are spaced from each other along the path of saidradiant energy.
 5. The invention as set forth in claim 4, wherein saidphotoelectric element and said photoemissive element are also laterallyoffset from each other, said photoemissive element being disposed aheadof said photoelectric element in the direction of propagation of suchenergy.
 6. Photoelectric detector apparatus which comprises: a. aphotoelectric transducer, b. said transducer being disposed directly toreceive illumination to be detected, c. a photoemissive element inspaced relationship with said photoelectric transducer, d. said elementalso being disposed directly to receive illumination to be detected, ande. electron optical means operative to direct electrons resulting fromsaid illumination to said transducer for augmenting the effect of saidillumination at said transducer.
 7. The invention as set forth in claim6 including means for rendering said electron optical means operativewhereby to operate said apparatus selectively in augmented and normalmodes.
 8. The invention as set forth in claim 6 wherein said electronoptical means includes means for accelerating the electrons emitted bysaid photoemissive element so that said electrons bombard saidtransducer to induce conduction therein.
 9. The invention as set forthin claim 8, wherein said electron optical means further includes meansfor converging said electrons into a beam focused at said transducer.10. The invention as set forth in claim 9 wherein said electron opticalmeans further includes means for deflecting said beam across saidtransducer.
 11. The invention as set forth in claim 9 wherein saidtransducer is a semiconductive photodiode.
 12. The invention as setforth in claim 10 wherein said transducer comprises a substrate disposedin the path of said beam and an array of semiconductive photodiodesdisposed in side by side relationship on said substrate.
 13. Theinvention as set forth in claim 6 wherein said photoemissive element isa photocathode, and said electron optical means, includes means forapplying voltage to said photocathode to make said photocathode morenegative than said transducer.
 14. The invention as set forth in claim13, wherein said electron optical means includes a plurality ofelectrodes disposed in spaced relationship between said photocathode andsaid transducer, and means for applying negative voltages ofsuccessively higher magnitude to said electrodes with the highest ofsaid voltages being applied to the one of said electrodes closest tosaid photocathode.
 15. The invention as set forth in claim 14 whereinsaid transducer has a surface, and wherein said electron optical meansincludes a coil disposed around said electrodes for converging saidelectrons from said photocathode into a beam focused at said transducersurface.
 16. The invention as set forth in claim 14 wherein saidelectron optical means includes a pair or array of permanent magnets,spaced laterally from each other, said transducer and said photocathodebeinG disposed adjacent opposite ends of the region between saidmagnets, said magnets defining a focussing field for electrons emittedby said photocathode.
 17. The invention as set forth in claim 16,wherein said photocathode is disposed along an axis extendinglongitudinally through said region, said photocathode being offset fromsaid axis.
 18. The invention as set forth in claim 17 including a mirrordisposed between said photocathode and said transducer and offset fromsaid axis, said mirror reflecting said illumination to saidphotocathode.
 19. The invention as set forth in claim 18 wherein saidtransducer has photoconductive material having a surface, and whereinsaid electron optical means includes means for scanning said focussedelectrons over said surface.
 20. The invention as set forth in claim 6further comprising an envelope having a window for the passage of saidillumination at one end thereof, said transducer being disposed at theend of said envelope opposite from said window, said photoemissiveelement being a photocathode disposed on the inner surface of saidwindow, said electron optical means comprising an electron gun withinsaid envelope.
 21. The invention as set forth in claim 20 wherein saidphotocathode is a layer of photoemissive material.
 22. The invention asset forth in claim 21, wherein said layer is partially transparent tosaid illumination.
 23. The invention as set forth in claim 21, whereinsaid transducer is located on the axis of said envelope and saidphotocathode is disposed upon a portion of said window which islaterally offset from said axis.
 24. The invention as set forth in claim21, including first lens means for focussing said illumination at saidtransducer, and second lens means for focussing said illumination atsaid photocathode.
 25. The invention as set forth in claim 20, furthercomprising deflection coil means encompassing said envelope, andfocussing coil means also encompassing said envelope.
 26. The inventionas set forth in claim 25, wherein said transducer is an array ofsemiconductive photodiodes, and leads to each of the diodes in saidarray extending from said diodes through said envelope at said one endthereof.
 27. The invention as set forth in claim 20 wherein saidelectron gun comprises a plurality of conductive electrodes disposedwithin said envelope, said electrodes being coaxial with the axis ofsaid envelopes, one of said electrodes being disposed upon said windowand in contact with said photocathode, said electrodes being spaced fromeach other in a direction longitudinally along said axis, means forapplying a first negative potential to said one electrode and negativepotentials of successively lower magnitude than the magnitude of saidone potential to the other ones of the other of said electrodes whichare spaced successively closer to said transducer.
 28. The invention asset forth in claim 27 including a cylinder of encapsulating materialsurrounding said envelope.
 29. The invention as set forth in claim 28including bar magnets encapsulated in said material and diametricallydisposed in the direction of the axis of said envelope and diametricallyopposite each other on opposite sides of said axis.
 30. The invention asset forth in claim 29 wherein said photocathode is disposed to one sideof said envelope, a mirror disposed within said envelope and spaced awayfrom said window toward said transducer and to the opposite side of saidenvelope for reflecting illumination to said photocathode, said barmagnets being tilted with respect to the axis of said envelope forestablishing a magnetic focussing field for focussing electrons emittedfrom said photocathode at said transducer.
 31. The invention as setforth in claim 20 including a means for focussing electrons emitted fromdifferent portions of said photocathode upon said transducer.
 32. Theinvention as set forth in claim 31 wherein said focussing meanscomprises deflection and focussing coil Means surrounding said envelope.33. The invention as set forth in claim 31 including means for providingsaid portions with different spectral characteristics.
 34. The inventionas set forth in claim 33 wherein said spectral characteristic providingmeans comprises filters interposed in the path of illumination to saidphotocathode.